A 72-year-old male with major depressive disorder and a 15-pound unintentional weight loss over 3 months is started on mirtazapine 15 mg at bedtime. After 4 weeks, his appetite has improved and he has regained 6 pounds, but his depressive symptoms are only partially remitting. The PMHNP increases the dose to 30 mg at bedtime. At his next follow-up, the patient reports a surprising observation: he feels noticeably less sedated on 30 mg than he did on 15 mg and is now having some difficulty falling asleep. Which of the following best explains this paradoxical reduction in sedation at the higher dose?